Tire pressure sensor for motor vehicles

ABSTRACT

The tire pressure sensor for motor vehicle has a reference pressure chamber (19), which is constructed as a pressure switch, including a contact diaphragm (22) and a contact pin (23). The reference pressure chamber (19) is closed toward the rim side by a ring diaphragm (28) connected at its inner rim with the housing of the pressure switch and at its outer rim (29) with the contact diaphragm (22) via a mass ring (26) for the purpose of shifting the threshold value of the pressure switch in a speed-dependent manner.

BACKGROUND OF THE INVENTION

The invention is directed to a tire pressure sensor for motor vehicles.

A tire pressure sensor for sensing the pressure in a tire of a motorvehicle is known. This tire pressure sensor comprises a housing fixed ona wheel rim of a vehicle wheel, on which a tire whose pressure is to bemeasured is mounted, and a pressure switch which is mounted at thecircumference of the wheel rim extending in a radial direction into theinterior of the tire. The pressure switch includes a reference pressurechamber, which is closed on a side closet the tire interior by anelectrically conducting contact diaphragm, which is arranged parallel tothe tread of the tire and which respect to the housing, according to therelative tire pressure and is lifted according to speed of the tire by amass, which is movable in a radial direction by centrifugal force forthe purpose of speed-dependent raising of the switching threshold of thepressure switch.

Tire and vehicle manufacturers recommend increased tire pressures forhigher vehicle speed. This can be taken into account in that theswitching threshold is adjusted to the greatest required pressure byadjusting the pressure in the reference pressure chamber. This resultsin a loss in comfort at low speeds (compare DE-OS 32 43 845). On theother hand, if the tire pressure sensor is adjusted to the tire pressurerecommended in the lower speed range at full load, the required highertire pressure at high speeds is not monitored and the tire pressuremonitoring device gives a false sense of security, which is dangerousprecisely in this speed range.

It is known from DE-OS 26 26 475 to provide the diaphragm of thereference pressure chamber of a tire pressure sensor with a solid pistonwhose front side is contacted by the tube of the tire to be monitored,which presses in the diaphragm against the pressure in the referencepressure chamber, when there is sufficient air pressure in the tire, tothe extent that an electrical contact in the reference pressure chamberis closed It is further mentioned in DE-OS 26 26 475 that centrifugalforces at the piston of the diaphragm advantageously reduce the forceexerted on the diaphragm by the tire pressure at increasing speed, sothat the tire pressure must be increased at higher speeds in order tokeep the switching contact closed. However, it is disadvantageous thatthe piston which is used as added mass for the diaphragm is covered bythe tube of the tire, so that the centrifugal forces acting on it act onthe reference pressure chamber only in part and in an undefined mannerfor raising the switching threshold. It is therefore not possible withthis solution to raise the switching threshold by a defined andreproducible amount at a determined speed relative to the stationarystate, since the speed-dependent raising of the switching threshold cannot be determined beforehand in this device.

SUMMARY OF THE INVENTION

On this basis, the object of the invention is to construct a tirepressure sensor with a speed-dependent shifting of the threshold valuein such a way as to provide an increased, reproducible switchingsensitivity and an improved coupling of the temperature of the referencepressure chamber to the temperature in the tire.

This object is met, according to the invention, the reference pressurechamber is closed toward the rim side (i.e. its peripheral edge) by aring diaphragm connected at its inner rim with the housing of thepressure switch and at its outer rim with the contact diaphragm via amass ring. Due to the intermediate ring diaphragm, the mass ring,generally a metal ring, can shift radially outward as a function of thecentrifugal force acting on it and accordingly as a function of thedriving speed. The state of equilibrium of the forces acting on thecontact diaphragm in the opposite direction is shifted toward highertire pressures, so that a shifting of the switching threshold value ofthe pressure switch toward higher pressures is effected. The pressureswitch accordingly responds promptly to a drop in the tire pressureprecisely at high speeds, when this is particularly important.

Since the tire pressure chamber projects practically in its entiretyinto the space enclosed by the tubeless tire as a result of thisconstruction and is accordingly surrounded by the tire air, a very goodtemperature compensation is achieved between the temperature of the airin the interior of the tire and the gas in the reference pressurechamber, so that the particularly sensitive switching behavior, which isa result of the construction anyway, is further improved.

It is advantageously provided that the mass ring is welded with the ringdiaphragm and the contact diaphragm. A durable connection and a hermeticsealing of the reference pressure chamber is accordingly achieved.

The radial width, thickness or material of the ring diaphragm, as wellas the mass of the mass ring, are selected in such a way that a desiredshifting of the speed-dependent switching threshold is achieved. Theshifting of the switching threshold can thus be determined beforehand bymeans of different parameters which are in part completely independentof one another, so that it is possible to produce defined characteristiclines of the shifting of the threshold value as a function of the speed,depending on the specific application and on the respectivemanufacturer's allowances. In so doing, the spring characteristic of thering diaphragm, which acts as plate spring, can also be utilized inparticular in a controlled manner according to the known calculations.The speed-dependent arching of the contact diaphragm, which is to beinfluenced by means of the geometry and material of the latter, canlikewise be determined along with this.

BRIEF DESCRIPTION OF THE DRAWING

Further features, advantages and details of the invention follow fromthe following description of a preferred embodiment example with the aidof the drawing.

FIG. 1 is a cross sectional view through a tire and the radial outerportion of the respective rim with a schematic side view of a pressuresensor inserted in the latter;

FIG. 2 is a cross sectional view through the reference pressure chamberarea of a pressure sensor, according to the invention, in a stationarywheel; and

FIG. 3 shows a view, corresponding to FIG. 2, in a wheel turning at ahigher speed.

DETAILED DESCRIPTION OF THE INVENTION

A motor vehicle wheel is shown in FIG. 1 in reduced scale and isdesignated by 10. It shows a tire 11 which is fastened on a wheel rim12. A tire pressure sensor 13 is fastened radially with respect to thewheel axis in a threaded bore hole 14 of the wheel rim 12 at an inwardlydirected portion of the wheel rim 12. A signal pick-up 15 is fastened ata slight distance from the latter in the area of the tire pressuresensor 13 at the wheel suspension of the vehicle, not shown; the frontside of the signal pick-up 15 is directed toward the tire pressuresensor 13 and feeds a high-frequency oscillation into the tire pressuresensor 13 as the latter passes by during every revolution of the vehiclewheel 10. The pick-up 15, together with the pick-ups of the remainingvehicle wheels, is connected to an evaluating circuit 16 with aconnected warning display 17 which gives a warning signal when the airpressure in one of the tires is too low. Such a pressure sensor isdescribed in more detail e.g in DE-OS 35 23 774.

The portion of the pressure sensor 13 facing the interior 18 of thetire, particularly the reference pressure chamber 19, is shown in moredetail in FIGS. 2 and 3. It is not shown in particular how the housing20 of the pressure sensor 13 is constructed and fixed in the rim 12.

The reference pressure chamber 19 comprises a diaphragm 22 which ismetallized at least on its inside 21 and which cooperates as a pressureswitch with a metal pin which acts as the switching contact 23 and isfastened in an upper cover 24 of the housing 20 so as to be insulated.

The outer rim 25 of the diaphragm 22 is welded at 27 with a mass ring 26consisting of metal.

The mass ring 26 is welded at its underside to the outer peripheral rim29, with a ring diaphragm 28 consisting of high-grade steel and having athickness of 0.3 mm. The ring diaphragm 28 is connected with its innerrim 29 the cover 24 of the housing along an annularly extendingprojection 30 so as to be sealed, so that a reference gas pressureadjusted in the interior space 31 of the reference pressure chamber 19,formed in this way, is maintained.

The contact diaphragm 22 comprises e.g. a high-grade steel layer havinga thickness of 0.2 mm and is welded with the mass ring 26 by a laserbeam. The contact pin 23 can carry a contact ball consisting of gold atits front end, which contact ball is not shown in detail in the drawing.The interior space 31 of the reference pressure chamber 19 is filledwith air or nitrogen with the required reference pressure in a mannerknown per se via a filling duct, not shown. After the reference pressurechamber 19 is filled, the filling duct is closed so as to be tightagainst pressure. This final state is shown in FIGS. 2 and 3.

In the rest state, shown in FIG. 2, the pressure switch formed by thediaphragm 22 and the contact pin 23 functions in a conventional mannerin such a way that the diaphragm 22 is raised when there is a drop inpressure in the interior 18 of the tire because of the gas pressure inthe interior space 31 of the reference pressure chamber (FIG. 2, attop), so that the conductive connection between the metallized inside 21of the diaphragm 22 and the contact pin 23 is interrupted and a pressuredrop signal can accordingly be detected. The diaphragm 22 according toFIG. 2 contacts the contact pin 23 when there is sufficient tirepressure.

FIG. 3 shows the behavior of the pressure switch when the wheel 10rotates at a relatively high speed at the same tire pressure as in FIG.2. Due to the centrifugal force, shown by the arrow 32, the mass ring 26is drawn outward as are the contact diaphragm 22 and the ring diaphragm28, so that the diaphragm 22 is lifted from the contact pin 23. A highertire pressure must now be provided in order to adjust the state ofequilibrium shown in FIG. 2, which higher tire pressure acts on thecontact diaphragm 22 in a direction opposite the arrow 32, so that ashifting of the switching threshold toward the higher pressures isachieved in this way. With a diaphragm diameter of 20 mm, the entirearching of the diaphragms 22 and

28 at high driving speeds amounts to only approximately 50 μm, whichcorresponds to a switching threshold increase of approximately 0.5 bar.In so doing, according to the example in FIG. 3, the ring diaphragm 28is lifted by means of the mass ring 26 approximately twice as much asthe contact diaphragm 22 because of its inherent mass. In this case, thearching of the contact diaphragm 22 is also consequently to be takeninto account in the raising of the switching threshold. On the otherhand, if the speed-dependent raising of the switching threshold isdecisively effected by means of a corresponding dimensioning of the ringdiaphragm 28 and mass ring 26, the arching of a contact diaphragm 22 oflow mass can possibly be ignored.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofstructures differing from the types described above.

While the invention has been illustrated and described as embodied in atire sensor for a motor vehicle, it is not intended and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:
 1. In a tire pressure sensor for a motor vehicle comprising ahousing and a pressure switch which extends at the circumference of awheel rim of a vehicle wheel in a radial direction toward an interior ofa tire mounted on said wheel rim, said pressure switch comprising aswitching contact fixed relative to said housing and a referencepressure chamber provided with an electrically conducting contactdiaphragm closing said pressure chamber on a side of said pressurechamber closest to the interior of the tire and positioned substantiallyparallel to the wheel rim, said contact diaphragm cooperating with saidswitching contact as a function of relative tire pressure, so that saidconducting contact diaphragm is lifted as a function of speed of saidtire by a mass which is movable in a radial direction by centrifugalforce for speed-dependent raising of a switching threshold of thepressure switch, the improvement wherein the reference pressure chamber(19) is closed on another side of said pressure chamber opposite saidside closest to the interior of the tire by a ring diaphragm (28) havingan outer peripheral rim (29) and an inner rim (29'), said ring diaphragm(28) being connected at the inner rim with the housing of the pressureswitch and at the outer rim with the contact diaphragm (22) via a massring (26).
 2. In a tire pressure sensor for a motor vehicle comprising ahousing attached to a wheel rim of the motor vehicle and a pressureswitch mounted on the wheel rim, said pressure switch extending radiallyinto an interior of a tire mounted on the wheel rim, said pressureswitch comprising a switching contact mounted in a fixed positionrelative to said housing and a reference pressure chamber provided withan electrically conducting contact diaphragm closing the pressurechamber, on which said switching contact can contact depending on arelative tire pressure in the tire, said electrically conducting contactdiaphragm closing said pressure chamber on a side of said pressurechamber closest to the interior of the tire and arranged substantiallyparallel to the wheel rim, so that the conducting contact diaphragm islifted depending on a speed of the tire by a mass which is movable in aradial direction by centrifugal force for speed-dependent raising of aswitching threshold of the pressure switch, the improvement comprising amass ring (26) and a ring diaphragm (28) having an outer peripheral rim(29), said outer peripheral rim (29) of the ring diaphragm beingattached to the electrically conducting diaphragm via the mass ring,said ring diaphragm also being attached to said housing and said ringdiaphragm acting to close said pressure chamber on another side of saidpressure chamber opposite said side closest to the interior of the tire,so as to provide a comparatively increased switching sensitivity andcoupling of a temperature of the reference pressure chamber to a tiretemperature.
 3. The improvement according to claim 2, wherein said ringdiaphragm is attached to said housing in the vicinity of an inner rim ofsaid ring diaphragm.